Which of these numbers is prime? ${9,\ 31,\ 35,\ 81,\ 98}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 9 are 1, 3, and 9. The factors of 31 are 1 and 31. The factors of 35 are 1, 5, 7, and 35. The factors of 81 are 1, 3, 9, 27, and 81. The factors of 98 are 1, 2, 7, 14, 49, and 98. Thus, 31 is a prime number.